To study an organ, such as a human brain, a three-dimensional image, or “volume scan,” may be taken. An exemplary volume scan may be taken using Magnetic Resonance (MR) Imaging. The result of a volume scan may be considered to be volume data. For a review of the basics of MR Imaging, see Joseph P. Hornak, The Basics of MRI, 1997 (available at www.cis.ritedu/htbooks/mri), the contents of which are hereby incorporated herein by reference. In particular, see the section of Chapter 12 entitled Volume Imaging (3-D Imaging). For a standard database of MR volume data examples, see www.bic.mni.mcgill.ca/brainweb. The volume data may be considered to be organized in slices. Three types of slices are typically considered particularly useful, including axial slices taken normal to and along a vertical axis in an axial plane, which divides the brain into top and bottom portions. The slices also include coronal slices taken normal to and along a longitudinal axis in a coronal plane, which divides the brain into anterior and posterior portions. The slices further include sagittal slices taken normal to and along a transverse axis in a sagittal plane, which divides the brain into left and right portions. Typically, each slice is considered to be a two dimensional array of pixel intensity values. However, a quality other than intensity (e.g., hue, saturation, etc.) may be associated with a pixel. The analog of a pixel in three dimensions is called a voxel. A voxel may be considered to have a size in each of three directions.
In human brain anatomy, two cerebral hemispheres may be identified as well as an interhemispheric fissure, which is a longitudinal furrow in the midline between the two cerebral hemispheres. A plane, called the “mid-sagittal plane,” may be defined as the sagittal plane passing through the interhemispheric fissure of the brain. The mid-sagittal plane typically contains less cerebral structure than a sagittal plane through any part of the rest of the brain in the vicinity of interhemispheric fissure.
When a volume scan of a given brain is taken, the position of the given brain within the co-ordinate system of the volume data depends on the position of the patient's head during the acquisition of the volume scan image. It is known that the volume data may be analyzed to determine a location for the mid-sagittal plane for the brain volume data. Determining the location for the mid-sagittal plane allows for the subsequent determination of a function to re-orient the mid-sagittal plane to a predetermined location. The re-orientation function, which may include translation and rotation, may then be applied to all of the brain volume data to orient the brain volume data in a preset co-ordinate system for further analysis.
Previously developed methods for determining the location for the mid-sagittal plane are based on the extraction of symmetry lines in axial or coronal slices of the volume data and use a priori information about slice direction. The basic classes of methods include a class of methods that are based on the interhemispheric fissure and a class of methods that are based on a symmetry criterion.
The basic hypotheses underlying the class of methods that are based on the interhemispheric fissure include a hypothesis that the interhemispheric fissure of the brain is roughly planar and a hypothesis that the interhemispheric fissure provides a good landmark for further volumetric symmetry analysis. In this class of methods generally, the fissure is identified as a segmented curve in MR images. Then, a three dimensional plane is found using an orthogonal regression from a set of control points representing the segmented curve.
The theory behind the methods based on a symmetry criterion is that the mid-sagittal plane maximizes the similarity between the brain image and its reflection, i.e., that the mid-sagittal plane is the sagittal plane with respect to which the brain exhibits maximum symmetry. Most of the methods based on symmetry share a common general scheme. First, an adequate parameterization is chosen to characterize any plane of the three-dimensional Euclidian space by a vector composed of a few coefficients. For each selected plane in a set of possible planes, an adapted similarity measure (symmetry criterion) is determined for the original three-dimensional brain image and a three-dimensional reflection image, where the reflection image is determined with respect to the selected plane. Then, the set of possible planes is searched to find the plane having the maximum adapted similarity measure. The chosen symmetry criterion is often the cross correlation between the intensities of voxels in the two three-dimensional images.
Unfortunately, known methods may include such drawbacks as a requirement for some a priori information, such as whether a given set of slices is oriented in an axial plane, a coronal plane or a sagittal plane. Additionally, the methods may be considered time consuming, orientation dependent and limited to small tilts in the data.
Clearly there exists a need for a method of determining the location of a mid-sagittal plane in a three-dimensional brain image that obviates a requirement for a priori information and overcomes other shortfalls of the previously developed methods.